Applications of ultrasound transducers include imaging, cleaning, surgical instrumentation, nondestructive testing, sonar, and the like. In particular, ultrasound imaging of the human body is a common medical technique.
Ultrasound transducers are widely used to image subsurface features (e.g. in the human body). An ultrasound beam is reflected from any discontinuities in the acoustic impedance of the sample. The reflected ultrasound waves return to the transducer where pressure variations are converted into an electrical signal. Ultrasound imaging is potentially inexpensive, especially compared to alternative technologies such as magnetic resonance imaging and computerized tomography. Current abdominal transducers and arrays typically operate in the 1-5 MHz frequency range, while specialty single-element transducers for detection of skin and eye ailments range from 30-100 MHz.
For imaging applications, an array of transducers is desirable, such as a one-dimensional or two-dimensional array. Conventional transducer arrays are fabricated by dicing bulk piezoelectric ceramics or single crystals with a diamond saw. Realistic machining tolerances limit the kerf (gap spacing between adjacent transducer elements) to >40 microns. The element spacing is typically lambda/2, where lambda is the acoustic wavelength, so that current transducer fabrication technologies limit transducer arrays to frequencies of less than about 20 MHz. Lateral resolution is proportional to wavelength and inversely proportional to transducer or array aperture. Thus, the higher the transducer frequency, the higher the lateral resolution.
A single ultrasound transducer may typically comprise a piezoelectric material, first and second electrodes positioned to apply an electric field to the piezoelectric material, a backing material, and a matching layer. A backing layer can be used to stop sound waves launched from the rear of the transducer from reflecting back and interfering with outgoing signals. A matching layer improves coupling of ultrasound energy between the transducer and the target material. The transducer typically has a resonance frequency at which the coupling coefficient is highest. In many applications the resonance frequency is determined mainly by the thickness of the piezoelectric element.
Beam steering generally requires that the transducer pitch be on the order of the ultrasound wavelength within the propagating medium to avoid grating lobe artifacts. Previous approaches have included laser micromachining of materials, however this approach has various problems including ceramic degradation at powers required for reasonable process time. Also, the kerf spacing (gap spacing between adjacent transducer elements) is preferably less than half the ultrasound wavelength to avoid lateral coupling between transducer elements.
A 50 megahertz phased array capable of electronic steering and focusing would require transducer elements with a 15 micron pitch separated by 5 micron kerfs. Such small kerfs cannot presently be achieved using a mechanical dicing technique. Current manufacturing techniques cannot achieve the frequency range of 50 megahertz to 1 gigahertz. However, there are many applications for higher frequencies, for example to obtain higher resolution images.
Hence new approaches are desirable to obtain improved high frequency ultrasound transducer arrays.